It's funny. Normally as actors grow older, they grow out of their roles, looking less like the character. But Hugh, and Halle (to a slightly lesser extent) look more like Wolverine and Storm than ever.

actually the cage fight was one of the last scenes they filmed which allowed time for hugh to bulk up. So when you see his physique at the beginning of the movie the audience believe that he's in that shape for the whole movie

__________________X-MENAlone, You Are Mighty
Together, You Are LEGENDS!- PROFESSOR CHARLES XAVIER

I'm sure it's all part of the publicity plan. The camera filming Hugh can't have been furtive.

Not part of publicity - you probably have no idea all the photos/videos snapped of Hugh. There was a video from today of him walking his dog and the dog got into a barking tiff with another dog. And there's always photos of him taking his kids to school. And this is not the first time he's been snapped working out from the outside looking in. He's followed by the paps in NYC all the time, especially since they know where he lives.